1. Field of Invention
The invention relates to the general field of piezoelectric ceramics and in particular to a piezosensitive device with variable, adjustable polarization. Such a device can be used, for example, in sensors of the electromechanical transducer type used in sonar antennas. These transducers can be hydrophones; accelerometers; hydrostatic, deformation, force, or temperature sensors; or complex systems that can combine these functions. These devices can also be used in the air, in microphones, accelerometers, teleprinter or computer keyboards, heat detectors, or piezochromic coatings.
As used herein, a "piezosensitive material" means a material that is sensitive to pressure and having piezoelectric and/or piezoresistive and/or piezopolar properties. Also, a "piezosensitive device" is a device that uses such a piezosensitive material.
2. Description of Related Art
Numerous piezoelectric materials exist, particularly composites, used in industry. For example, U.S. Pat. No. 2,420,864 describes a method for manufacturing a piezoelectric material having a plastic matrix and a piezoelectric monocrystal. The plastics referred to are celluloids, cellulose acetates, chlorine-containing rubbers, phenol formaldehydes, phenol-furfurol resins, acrylates, methacrylates and polystyrenes, while the monocrystals are chosen from potassium sodium tartrate, tourmaline, saccharose and tartaric acid. These materials are designed to be used solely in hydrostatic mode, namely by exposing their entire surface to the incident acoustic pressure. However, these monocrystals have very low hydrostatic constants and are hence inefficient.
Monocrystals have been gradually replaced by perovskite ceramics, particularly lead titanate zirconate or barium titanate, when high transmission powers are used. However, on reception, their hydrostatic constant is less than that of monocrystals such as lithium sulfate. Also, their application is limited to specific compression modes belonging to types 33, 31, or 32.
U.S. Pat. No. 4,977,547 describes a composite piezoelectric material made of ceramic powder and polymer. This powder is a sintered mixture of lead titanate and double iron and bismuth oxide whose average grain size is 5 microns, while the percentage by weight of double iron and bismuth oxide is between 50 and 80%.
U.S. Pat. No. 4,407,054 describes a composite piezoelectric material composed of a prepolymer such as a mixture of castor oil and ricinoleate isocyanate and a piezoelectric material such as quartz, lead titanate zirconate, tourmaline, tartaric acid, or lithium sulfate.
U.S. Pat. No. 4,868,856 describes a composite piezoelectric material composed of a polymer made from polyurethane and a piezoelectric material such as tartaric acid, a monosaccharide, a disaccharide, or a carboxylate.
Products known as "electrets," obtained by subjecting waxes, liquefied by heating, to a strong electrical field during their cooling phase, are known to have pronounced, permanent pyroelectric and piezoelectric properties.
All of the above-described materials have numerous drawbacks, particularly when used in hydrophones. Their polarization or, more generally, their piezoelectric properties are "frozen" at the time they are manufactured so that their operating mode is unique or specialized (hydrostatic mode, mode 33, 31, 15, etc.). As an example, a composite obtained by mixing grains of tartaric acid or lithium sulfate with a resin that has not yet been polymerized, then polarizing the grains by one of the known methods, can be used only in a hydrostatic mode, and its piezoelectric properties are in principle invariable, or vary slowly over time.
French Patent No. 2719181 describes a material whose polarization, and hence whose piezoelectric properties, or more generally whose piezosensitive properties, can be varied at will by applying an electrical field when the transducer containing this material, or the antenna containing the transducers using this material, are in operation.
When, for example, an antenna breaks down, a material according to the invention can be used to remedy the situation by repairing the antenna. Hence, it is not necessary to repair the receiver--something that is not always possible, or increases the cost of the receiver.
For this purpose, a device according to this invention comprises one or more piezosensitive materials made of a matrix of a polymer or a dielectric, non-piezoelectric, or slightly piezoelectric organic product, and particles, and a voltage supply for applying a polarizing voltage, whose value is adjustable, to this material or these materials.
The particles, such as fibers, powders, or fragments of objects or materials, are, as the case may be, piezoelectric, slightly piezoelectric, or even non-piezoelectric. Particles may be derived wholly, or in part, from at least one biological material or sheets or slabs of piezoelectric plastics prepolarized in the direction of their thickness or non-prepolarized, or microcrystals.
As an example, the biological material can be chosen from the oligosaccharides, homopolysaccharides, heteropolysaccharides, glycosaminoglycans, proteoglycans, terpenes, steroids, amino acids, oligopeptides and polypeptides, proteins, nucleotides, nucleic acids, urea, or natural silks.
Also, these particles may be derived wholly or in part from microcrystals or fibers of ammonium tartrate, potassium tartrate, ethylenediamine tartrate, dipotassium tartrate, sodium chlorate, sodium bromate, nickel sulfate hexahydrate, iodic acid, benzophenone, hydrated or dehydrated lithium gallate, polyvinyl difluoridene, polyvinyl chloride, a copolymer of vinyl acetate and vinylidene cyanide, a copolymer of vinylidene and trifluorethylene, polyvinyl fluoride, nylon, polyacrylonitrile, polyparaxylylene, polybichloromethyloxetane, aromatic polyamide, polysulfone, cyanoethylcellulose, monosaccharides, disaccharides, brushite, monetite, or carboxylic acid salts.
According to one preferred embodiment, the matrix of polymer or the dielectric, non-piezoelectric, or slightly piezoelectric dielectric organic product is chosen from polystyrenes, polysulfones, polycarbonates, polypropylenes, polyethylene, polyethylene terephthalates, polyphenylene oxides, polyurethane elastomers, polyvinylcarbazoles and polyvinylidene fluorides, polyacrylate or polymethacrylate esters, polyvinyl chlorides, polyvinylidenes, acrylonitrile or methacrylonitrile polymers, polyvinyl acetates, celluloids, cellulose acetates, polyvinylpyrrolidones, cellulose polymers, phenolformaldehydes, phenol-furfural resins, soluble polyimide polymers, epoxy resins, polymerizable resins, natural rubbers, synthetic rubbers, silicone resins, carbinol or bone glues, polystyrols, shellac, or vinylidene trifluorethylene copolymers.